Methods of and apparatus for electrostatic printing

ABSTRACT

Printed matter is recorded on dielectric paper by depositing an array of electrostatic charges on one surface of the paper at a potential level lower than the ionization potential and then raising the potential level of the charges higher than their ionization level according to a selected pattern with stylet-like electrodes. The selected pattern causes a developer to adhere to the dielectric paper in accordance with the pattern, thereby making the pattern visible.

United States Patent Quang et al.

METHODS OF AND APPARATUS FOR ELECTROSTATIC PRINTING Inventors: Pham KimQuang, Dieppe; Rene Gasdou, Neuville les Dieppe, both of FranceAssignee: La Cellophane, Paris, France Filed: Mar. 18, 1974 Appl. No.:452,346

Foreign Application Priority Data Apr. 3, 1973 France 73.11867 US. Cl346/74 ES Int. Cl. G03G 15/18 Field of Search 346/74 EL, 74 ES, 74 EB,

346/74 IB; l78/6.6 A

References Cited UNITED STATES PATENTS 3/1962 Schwertz 346/74 EW 51 Dec.30, 1975 Lloyd 346/74 ES Gundlach et al 346/74 ES PrimaryExaminerBernard Konick Assistant Examiner.lay P. Lucas Attorney, Agent,or FirmSherman & Shalloway [57] ABSTRACT Printed matter is recorded ondielectric paper by depositing an array of electrostatic charges on onesurface of the paper at a potential level lower than the ionizationpotential and then raising the potential level of the charges higherthan their ionization level according to a selected pattern withstylet-like electrodes. The selected pattern causes a developer toadhere to the dielectric paper in accordance with the pattern, therebymaking the pattern visible.

12 Claims, 3 Drawing Figures CON TROL LER LOW VOLTAGE GENERATOR U.S.Patent Dec. 30, 1975 3,930,257

24 26 CONTROLLER 27 b LOW VOLTAGE GE NERATOR 32 CONTROLLER LOW VOLTAG EGEN ERATOR FIG? METHODS OF AND APPARATUS FOR ELECTROSTATIC PRINTINGBACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention relates to electrostatic printing. More particularly, thepresent invention relates to methods of and apparatus for arrangingelectrostatic charges in a selected configuration on a sheet of paper orother dielectric material to be subsequently developed by a liquid orpowder developer.

2. Technical Considerations and Prior Art There are currently availableseveral methods of and apparatus for electrostatic printing. However,each of these currently practiced approaches has disadvantages. Forexample, a system which is used in printers which record the output ofcomputers deposits electrostatic charges on a sheet of paper having acoating of insulation by passing the sheet of paper between a pair ofelectrodes maintained at different potential levels. Generally, one ofthe electrodes is a mobile stylet which is brought to a predeterminedpotential by pulses generated therein with an electric signal generatingdevice. This particular system presents problems because, in order toobtain sufficient line density after development, it is necessary thatthe insulating layer be supported by a very good electrical conductor,and that the potential difference developed between the electrodes bequite high. Accordingly, the potential difference between the electrodesmust be between 400 and 500 volts, and often, depending on the nature ofthe insulating layer, support and spacing between the stylet and thedielectric sheet, the voltages may range from 600 to 1,000 volts. Thesehigh voltages require a good deal of electricity, and make it necessaryto take certain precautions to ensure safety. In addition, the practicalproblems raised by utilizing this particular process are complicated dueto the need for rapid switching devices.

In order to overcome the deficiencies of the aforementioned system orprocess, the prior art suggests placing two additional electrodes onopposite sides of the mobile stylet electrodes. These two electrodesamid voltages having a sign opposite to that of the stylet thereby raisethe potential level of the conductive support over which the paperpasses. By using this process, it is possible to operate with slightlylower voltages, but obviously this approach is merely remedial and addsto the complexity of the printer. In addition, as with theaforedescribed process, the use of the additional two electrodes stillrequires using a support for the paper or dielectric material which is agood electrical conductor. In addition, it is no mean task to easilymodify the polarity of the charges deposited on the dielectric materialwithout resorting to drastically increasing the complexity of theprinter.

An additional approach which has been envisioned is to deposit on aninsulating layer a prior electric charge having a level lower than thecritical voltage necessary for good production of images. This process,however, results in print-outs which have a poor background afterdevelopment.

SUMMARY OF THE INVENTION In view of the afore-mentioned deficiencies ofprior art devices, it is an object ofthe instant invention to forelectrostatic printing wherein printing is applied on dielectricmaterials mounted on supports which are not electrical conductors.

It is still an additional object of the instant invention to provide newand improvedmethods of and apparatus for electrostatic printing whereinthe printing is applied to dielectric materials mounted on supportswhich are only slightly conductive of electricity.

It is still a further object of the instant invention to provide new andimproveed methods of and apparatus for electrostatic printing whereinsupports for the dielectric materials being printed on easily acceptelectrostatic charges, the polarity of which can be easily reversed.

It is an additional object of the instant invention to provide new andimproved methods of and apparatus for electrostatic printers whereinprinters have stylets that use relatively low inscription voltages whichare capable of response times that are considerably shorter than theresponse times of prior art devices.

It is still a further object of the instant invention to provide new andimproved methods of and apparatus for creating an external field in thesupport for a dielectric layer being printed on by usingtribe-electricity or, rather, friction-induced electricity.

It is still another object of the instant invention to provide new andimproved methods of and apparatus for electrostatic printing whereinsaid methods and apparatus are relatively non-complex and relativelyinexpensive to manufacture and use.

It is a further object of the instant invention to provide new andimproved methods of and apparatus for electrostatic printing wherein themethods and apparatus utilize voltages levels in a way which isrelatively safe and, therefore, does not require extensive safetyequipment.

Still a further object of the instant invention is to provide new andimproved methods of and apparatus for electrostatic printing wherein thedielectric layer which is printed upon may be composed of readilyavailable, inexpensive paper having a conventional, readily available,dielectric surface.

In view of these and other objects, the instant invention is directed tomethods of and apparatus for electrostatic printing which include theconcepts of advancing a strip of dielectric material past a chargingstation in order to distribute an array of electrostatic chargesthereon, and then re-arranging the array of electrostatic charges tocorrespond to a selected image by advancing the dielectric material pastan electrode means maintained at a potential level different from thepotential level of the array. The resulting image is then developed byfixing it permanently on the dielectric material.

BRIEEDES CRIPTION OF THE DRAWINGS 3 of dielectric material prior toarranging the charges with a stylet type electrode;

FIG. 2 is a schematic view of a second embodiment of the inventionwherein electrostatic charges are placed directly on a strip ofdielectric material without first utilizing a belt of insulatingmaterial as an intermediary to deposit the charges; and

FIG. 3 is an enlarged cross-section of the type of paper being printedupon in accordance with the principles of the invention.

DETAILED DESCRIPTION Referring now to FIG. '1, there is shown oneembodiment of a printer in accordance with the instant invention,wherein negative charges are deposited on a belt 1 1 of insulatingmaterial, such as polyester film, by a corona-charging device 12. Thebelt 11 is trained around three rollers l3, l4 and 15, one of which isdriven and all of which are positively engaged by the belt. The belt isdriven by the powered roller to travel in the direction of the arrow 16so as to pass through a printing station, designated generally by thenumeral 17.

A supply of dielectric paper 18 is dispensed from a supply reel 19 andpasses over a roller 21 positioned opposite the roller 13, and a roller22 positioned opposite the roller 14, so as to travel in a paralleldirection with the belt 11 while in the printing station 17. The paper18 is positively driven by the rollers 21 and 22 at the same linearspeed that the belt 11 is driven. This may be accomplished by anyconvenient means, such as gearing the rollers 21 and 13 together and therollers 14 and 22 together so that these rollers each rotate at the samespeed.

While the dielectric paper 18 and belt 11 are advancing through theprinting station 17, the electrostatic charges 10 are transferred in aselected manner so as to record intelligence on the paper 18 by raisingthe level of potential of the charges to their ionization levelaccording to a selected pattern. When the ionization level is reached,the charges so raised will, in effect, transfer to the paper 18 in theselected pattern to record intelligence on the paper in the form ofletters, numbers and the like.

The selected transfer of the charges 10 from the belt 11 to the paper 18is accomplished by stylets 24 (only one of which is shown). Thesestylets are electrodes which are connected by leads 26 to a pole 27 of alow voltage generator 28. The pole 27 is opposite in polarity from thecharges 10 deposited by coronacharging device 12. In other words, if thecharges 10 are negative, the pole 27 will be positive, and it thecharges 10 are positive, then the pole 27 will be negative. Thegenerator 28 has another pole 30 which is grounded through lead 31, andis controlled by signals from a controller 32 impressed over a lead 34.Controller 32 lies outside the scope of the present invention, but maybe assumed to be the type that is used to control the prior artelectrostatic printers. Since the stylet 24 has an opposite polarityfrom the charges 10 due to the pole 27, when the charges 10 locatedopposite the stylet advance past the stylet, their level of potential israised. If the potential level of the charges 10 are quite close totheir ionization threshold, then the electric field at the tip of thestylet 24 will cause the charges to ionize if the potential level of thestylet, when added to the potential level of the charges, exceeds theioniza- 4 tion threshold. This phenomenon may be expressed as follows:

V Potential level of charges;

V Potential level of stylet;

V,-,,,, Potential level of ionization threshold;

Then V V s V (no transfer of charge 10 from belt 11 to paper 18).However,

V V V (results in a transfer of charges 10 from belt 11 to paper 18).

It should be understood that the stylets 24 may be arranged in anymanner which is convenient to cause transfer of the charges 10 from thebelt 11 to the paper 18 according to a selected pattern of intelligence.

There are known teachings in the prior art showing just how the stylets24 may be arranged, which include arranging the electrode in parallelbatteries to conform to various configurations, such as combs or thelike. Each electrode 24 may be connected to an associated generator by asuitable switching system of a conventional, well-known type. Theseaspects lie outside the scope of the present invention.

After the charges 10 have been transferred from the belt 11 to the paper18 in a selected manner, the intelligence recorded on the paper 18 isnot yet visible. In order to make the intelligence visible, the paper 18is carried around a conductive roller 36 and passed between theconductive roller and a developing roller 37. The developing roller 37rotates through a bath 38 of liquid developer which is transferred tothe paper 18 and fixed to the paper by a heater 39 before beingaccumulated on a take-up roll 40.

The conductive roller 36 serves to short-circuit the backside of thepaper 18 so that charges deposited thereon are removed from the backsidebefore the liquid developer 38 is deposited by the developing roller 37.Although the charges are removed from the backside of the paper 18, theyremain on the front side of the paper so that the intelligence recordedon the front side of the paper is not confused by developer which isadhered to the paper due to charges on the back of the paper.Consequently, the background is not darkened, and print on the front ofthe paper is clearly developed.

One of the objects of this invention is to have the stylets 24 operatewith a relatively low potential. In practice, this potential is on theorder of 50V. However, this potential may be regulated to be as high orlow as desired so long as, when added to the potential of the charges 10on the belt 11, the ionization thresh old of the charges will beexceeded.

Generally, the paper 18 may be considered a dielectric system in whichpaper forming an insulating layer 45 is coated with a standarddielectric layer or coating 46 of, for example, butadiene-styrenepigmented with zinc sulfide. The resin, of course, may be any othersufficiently dielectric polymer, such as one of the vinyl copolymers,acrylic resins, silicones, cellulose esters, etc. In order to pigmentthe resin, titanium, zinc, tin oxide, silica and/or any other knowpigment may be used. The most suitable dielectric paper are those whichhave volume resistances equal to or in excess of l0 ohms/cm /cm. Since,in this process, the charges are not placed on the paper by passage ofcurrent between two electrodes, but rather by injection through thesupporting paper or insulating layer 45, the paper 18 does not have tobe a good conductor of electricity. Consequently, ordinary paper may beused.

In practice, the potential level developed at the tip of the stylet 24is achieved by pulsing the stylet with pulses having a period of to 50micro seconds at a level, as mentioned previously, of 50V; whereas, inthe prior art, the pulses were for a period of a millisecond at apotential level of 600 to 1,000 V.

An additional embodiment of the present invention is shown in FIG. 2where the use of a belt of insulating material, such as the belt 11 ofFIG. 1, is dispensed with. In the embodiment of FIG. 2, a strip ofdielectric paper 18 is advanced from a supply roll 19' around a firstroller 21 and a second roller 22. A printing station 17' is disposedbetween the first roller 21' and the second roller 22. Instead ofdepositing charges first on an insulating belt, a corona-charging device12 is positioned in the printing station 17' at a location just oppositethe stylet 24'.

The corona-charging device 12 deposits charges on the paper 18 while thestylet 24' raises certain of these charges past their ionization levelat selected locations across the paper 18 to thereby record intelligenceon the paper 18'. As with the embodiment of FIG. 1, the stylet 24' isconnected to a pole 27 on a low voltage generator 28 which is oppositein polarity to the polarity of the charges deposited on the paper 18 bythe corona-charging device 12'. The low voltage generator 28 iscontrolled in the same way as the low voltage generator of the FIG. 1embodiment by using a controller 32. Downstream of the roller 22', aconductive roller 36' and development roller 37' cooperate with aheat-fixing device 39 to form an image on the paper 18' in the same wayas done with the embodiment of FIG. 1.

In practicing the invention according to the embodiment of FIG. 2, thecorona-charging device 12' may be placed at the back of the supportinglayer of the dielectric paper 18' at a distance ranging from 10 to 100mm. It has been found that a distance of to mm gives good results, andthat it is not necessary to place the corona-charging device 12' exactlyopposite the stylet 24. The corona-charging device 12' may be shiftedaround, provided that no metallic portion thereof comes into contactwith the back of the paper 18 in the zone located between thecorona-charging device and the stylet 24.

Depending on the characteristics of the dielectric layer coating thepaper 18 (which is similar to the dielectric layer coating the paper 18in the embodiment of FIG. 1) and the nature of the supporting paperlayer, the wires of the corona-charging device 12-12 are elevated to apotential of between 1,000 to 15,000 V. Generally, in practice apotential ofl ,000 to 6,000 V is used. It is found that potential mustbe increased as the distance between the corona 12 and the paper support18 is increased. The value of this potential, of course, must beregulated as a function of this distance between the corona and thepaper to produce an electric field which is less than the fieldnecessary to reach the ionization potential, but is as close to thethreshold of ionization potential as possible. Accordingly, the voltagethat must be applied to the stylet may be reduced to a range of 25 to V.In order to simplify the equipment of the control circuits as much aspossible, it is to ones advantage to use as low voltages as possiblewhich are of a sufficient level to elevate the voltage of the chargesdeposited by the corona past the ionization potential threshold.

In another embodiment of the invention, the external field may becreated by applying an insulating sheet of a film, such as polyesterfilm, which is uniformly charged to the backside of the dielectricsupport or paper. This dielectric sheet may be uniformly chargedbeforehand by a corona-charging device, such as those disclosed.

Instead of using a corona-charging device, such as 12 or 12', it ispossible to deposit charges on the paper by using tribo-electricity or,rather, friction-induced electricity. This may be accomplished by aconvenient tribo-electricity generating device which, for example, maybe a nylon brush applied against the back of the supporting layer of thepaper 18 and rotated at high speed in a direction opposite the advanceof the paper 18.

Another approach within the spirit of this invention is to raise thevalue of the field of charges deposited on the dielectric paper to alevel sufficient to produce a deposit of charges on the dielectric paperas soon as stylet 24' or 24 is brought close to the opposite surface ofthe paper. The distance between the stylet and paper may be reduced, inthis case, hundreds of millimeters. In this situation, the level of thefield in the paper should be higher than the ionization threshold forthe distance considered. With this embodiment, it is possible totransmit a document by telecopy, wherein the printing is formed at adistant station by advancing and retracting printing keys which haveprinting characters thereupon as instructed by remotely controlledelectric signals. Once transferred onto the dielectric surface, thedeposited charges may be developed by a liquid or powder developer, suchas in the embodiments of FIGS. 1 and 2, and then thermally fixed by wellknown methods.

What we claim:

1. An apparatus for electrostatic printing on a strip of material,wherein said material includes an insulating support layer with adielectric layer thereupon, the dielectric layer having an interiorsurface and an exterior surface, said apparatus comprising:

means for locating the material at a printing station;

means for depositing a uniform field of electrostatic charges in theinsulating support layer, wherein the charges have a level of potentialwhich is below a threshold ionization level;

stylet means for creating the charge image, said stylet means beinglocated in said printing station, wherein said stylet means is broughtto a predetermined level of potential substantially lower than that ofthe electrostatic charges and of opposite polarity, but high enough tocause ionization when added to the potential level of said charges tothereby transfer a charge image to the exterior surface of thedielectric layer; and

means for developing said charge image to make the image visible.

2. The apparatus of claim I, wherein the means for depositing theelectrostatic charges is located in the printing station at a positiongenerally opposite the stylet.

3. The apparatus of claim 1, wherein the means for depositing theelectrostatic charges deposits the charges on a belt of insulatingmaterial which transfers the charges upon subsequently passing throughthe printing station while positioned adjacent the dielectric material.

4. The apparatus of claim 1, wherein the stylet means includes aplurality of electrodes, each of which is selectively pulsed to thepredetermined level of potential by a low voltage generator.

5. A process of electrostatic printing on a strip of material, whereinsaid material includes an insulating support layer with a dielectriclayer thereupon, the dielectric layer having an interior surface and anexterior surface, comprising the steps of:

creating a uniform field of electrostatic charges within the supportlayer; ionizing a selected portion of the field of electrostatic chargeswith a potential of opposite polarity to the charges to transfer theselected portion of the field to the exterior surface of the dielectriclayer to thereby form a charge image thereupon; and

applying an electrostatic developer to the exterior surface of thedielectric layer to render the charge image visible.

6. The process of claim 5, wherein the electrostatic field within thesupport layer has a potential approaching the ionization potentialthereof and wherein the ionizing step is accomplished by elevating thepotential of only the selected portion above the ionization potential.

7. The process of claim 6, wherein the uniform field of electrostaticcharges is produced from charges previously deposited on an insulatingsurface.

8. The process of claim 6, wherein the uniform field of electrostaticcharges is deposited within the support layer by triboelectricity.

9. The process of claim 6, wherein the uniform field of electrostaticcharges is created by a corona charging device and the ionizing step isperformed by an electrode.

10. The process of claim 9, wherein the electrode is placed at a fixeddistance from the dielectric layer and is pulsed with low voltagesignals to effect ionization of the charges.

11. The process of claim 9, wherein the electrode is kept at zeropotential with respect to ground and wherein ionization of the chargesis effected by bringing the electrode into proximity with the exteriorsurface of the dielectric layer.

12. The process of claim 9, wherein the uniform field of electrostaticcharges is created in the support layer by positioning the coronacharging device adjacent to the support layer in opposed relation withthe electrode which is positioned adjacent to the exterior surface ofthe dielectric layer.

1. An apparatus for electrostatic printing on a strip of material,wherein said material includes an insulating support layer with adielectric layer thereupon, the dielectric layer having an interiorsurface and an exterior surface, said apparatus comprising: means forlocating the material at a printing station; means for depositing auniform field of electrostatic charges in the insulating support layer,wherein the charges have a level of potential which is below a thresholdionization level; stylet means for creating the charge image, saidstylet means being located in said printing station, wherein said styletmeans is brought to a predetermined level of potential substantiallylower than that of the electrostatic charges and of opposite polarity,but high enough to cause ionization when added to the potential level ofsaid charges to thereby transfer a charge image to the exterior surfaceof the dielectric layer; and means for developing said charge image tomake the image visible.
 2. The apparatus of claim 1, wherein the meansfor depositing the electrostatic charges is located in the printingstation at a position generally opposite the stylet.
 3. The apparatus ofclaim 1, wherein the means for depositing the electrostatic chargesdeposits the charges on a belt of insulating material which transfersthe charges upon subsequently passing through the printing station whilepositioned adjacent the dielectric material.
 4. The apparatus of claim1, wherein the stylet means includes a plurality of electrodes, each ofwhich is selectively pulsed to the predetermined level of potential by alow voltage generator.
 5. A process of electrostatic printing on a stripof material, wherein said material includes an insulating support layerwith a dielectric layer thereupon, the dielectric layer having aninterior surface and an exterior surface, comprising the steps of:creating a uniform field of electrostatic charges within the supportlayer; ionizing a selected portion of the field of electrostatic chargeswith a potential of opposite polarity to the charges to transfer theselected portion of the field to the exterior surface of the dielectriclayer to thereby form a charge image thereupon; and applying anelectrostatic developer to the exterior surface of the dielectric layerto render the charge image visible.
 6. The process of claim 5, whereinthe electrostatic field within the support layer has a potentialapproaching the ionization potential thereof and wherein the ionizingstep is accomplished by elevating the potential of only the selectedportion above the ionization potential.
 7. The process of claim 6,wherein the uniform field of electrostatic charges is produced fromcharges previously deposited on an insulating surface.
 8. The process ofclaim 6, wherein the uniform field of electrostatic charges is depositedwithin the support layer by triboelectricity.
 9. The process of claim 6,wherein the uniform field of electrostatic charges is created by acorona charging device and the ionizing step is performed by anelectrode.
 10. The process of claim 9, wherein the electrode is placedat a fixed distance from the dielectric layer and is pulsed with lowvoltage signals to effect ionization of the charges.
 11. The process ofclaim 9, wherein the electrode is kept at zero potential with respect toground and wherein ionization of the charges is effected by bringing theelectrode into proximity with the exterior surface of the dielectriclayer.
 12. The process of claim 9, wherein the uniform field ofelectrostatic charges is created in the support layer by positioning thecorona charging device adjacent to the support layer in opposed relationwith the electrode which is positioned adjacent to the exterior surfaceof the dielectric layer.